ANKARA Ibrahim Ozcan has worked at the fish market in Ankara's usually bustling Sakarya street for more than 30 years, and even he struggles to remember a time when the heart of the Turkish capital has been this quiet.

A suicide car bomb tore through a transport hub just a few hundred metres away on Sunday, killing 37 people and wounding dozens more. It was the third such attack in five months in the city, leaving many residents reluctant to venture out.

"Nobody has come for the last few days. Our sales are 60 percent down ... Especially young people aren't coming to Sakarya any more," said Ozcan, 57, his stall still packed with fish after what should have been a busy day's trade.

Tables spilling onto the pavements of Sakarya street would usually be packed with students and office workers at lunchtime and in the evenings. But two days after the blast, hardly a customer was being served in its cafes and bars.

"I've been here 10 years. I've never seen days this quiet. Normally you can't see an empty table at this time of day," Menderes Korkmaz, 42, who owns a kebab restaurant on the street, said in the early evening.

The government has said two members of the Kurdistan Workers Party (PKK) militant group, which has waged a three-decade insurgency for greater Kurdish autonomy in the southeast, were responsible for Sunday's suicide car bombing.

There has been no claim of responsibility, but if confirmed, the targeting of civilians in Ankara would mark a dangerous shift in strategy for a group that has in the past focused its attacks on the security forces in the southeast.

President Tayyip Erdogan has said the attack will not weaken Turkey's resolve in fighting terrorism. Warplanes hit PKK camps in northern Iraq hours after the blast, and clashes between the militants and the security forces have spread in the southeast.

There have been several security scares in Ankara and Istanbul amid fears of repeat bombings.

The German embassy sent an email to its citizens in Ankara on Tuesday warning of possible attacks on shopping malls, while police in Istanbul briefly closed one of the two bridges across the Bosphorus Strait to search a suspicious vehicle.

"My primary request to our citizens is that we be careful, cautious and alert, but do not allow ourselves to be panicked," Anadolu Agency quoted Istanbul Governor Vasip Sahin as saying on Wednesday.

Every possible measure was being taken "at the highest level" to ensure security, he said.

MORE VIOLENCE FEARED

Authorities in cities including Istanbul and Diyarbakir, the largest in the mainly Kurdish southeast, have banned Kurdish New Year (Newroz) celebrations on March 21. At the height of the PKK insurgency in the 1990s, the day was often marked by violent Kurdish protests, something many fear will be repeated next week because of the upsurge in fighting in the southeast.

"The government's efforts to block Newroz celebrations are neither legal nor legitimate. What is legitimate and right is celebrating Newroz, as it has been for thousands of years," the pro-Kurdish opposition HDP said in a statement on Wednesday.

"We will carry out our Newroz celebrations as we planned and announced. Our people will be in the streets and squares."

Newroz coincides with the spring thaw, a time in years past when PKK fighters re-entered Turkey from mountain hideouts in northern Iraq and violence escalated.

The HDP's co-leader Selahattin Demirtas warned earlier this month that clashes could pick up in the spring and spread to new urban centres, noting that both the PKK and the government had vowed to step up their campaigns.

It is a fear being felt beyond the southeast.

"The Kurdish issue has been there for a very long time ... the PKK has always been a danger. But it's the first time attacks like this are happening," said a retired bank employee in central Ankara, declining to give her name because of the sensitivities around discussing the Kurdish conflict.

"I've lived in this city for 30 years and I've never lived through a harder five months ... I'm looking at each passing car and wondering if it is loaded with bombs. I've told my kids to stay indoors."